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I have installed cement board and waterproof coated it. My next step is to install tile. I have a 4" access hole in the backer board for the rough-in valve per the instructions. The instructions are not clear whether the valve's trim sits on the backer board or tile.

To be clear, the "valve trim" I speak of is shown in the second picture below.

Option 1: Install valve trim onto the backer board

If the trim were to install onto the backer board, I'm guessing the trim's foam insert would compress to keep water from entering the backer board access hole. However, this seems weird to me for two reasons:

  1. The valve trim would need extra trim itself after tile is installed since there would be a gap between tile and the valve trim.

  2. This seems overly complicated since cutting tile to fit the valve trim is not always simple and then adding aforementioned extra trim.

Option 2: Install valve trim onto the tile

If the valve trim were to install on the tile, the only pitfall I see is the backer board access hole allowing water inside the wall. Assuming this is the option to go forward with, how would you seal the access hole given its size, but still have it be an access hole when you need it later?

Figure 1: Rough-in valve installed in backer board with 4" access hole

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Figure 2: Valve trim shown as if it were installed on backer board

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  • it should not be hard to freehand cut such a large hole in tiles using an 4" angle grinder with a diamond wheel.
    – Jasen
    Mar 29 at 0:27
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    It usually goes on the tile. The handles and final trim probably will only fit one way. Try them.
    – jay613
    Mar 29 at 3:23
  • If you’re that concerned (and you shouldn’t be), make a semicircular diverter bead on the top half of caulking at the edge of the tile, underneath the trim plate. Mar 29 at 12:03

3 Answers 3

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The shower valve trim will always go over the tile.

Usually the trim will have a foam backing ring that should compress against the tile and prevent water from getting behind it. If the tile is uneven or you do not feel comfortable with the seal of the valves built in foam gasket you can caulk around the trim with a latex caulk. we usually use white or clear for the caulking but it depends on your tile/ grout colors. If you ever need to access the ports on the valve behind the trim you simply cut the caulking with a razor knife and remove the trim.Simply clean off the caulking and re-seal the trim again after you make your adjustments to the valve.

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The trim goes over the tile and covers the hole. Caulk around the edge to seal it from the water. Leave a little spot at the very bottom uncaulked as a weep hole.

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  • This answer currently does not address the waterproofing concerns of the backer board access hole. Seemingly, water would go into the wall through the backer access hole and not all weep through the trim hole.
    – Scott Lin
    Mar 29 at 0:51
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    @Scott Lin, Your understanding of the situation is a bit skewered. Rarely if ever does "water" get behind the trim. If it does there has been a major malfunction in the total shower protection system. So forget about that. A weep hole is recommended at the bottom of the trim for any condensation. If it forms on the back of the metal trim in an amount that it runs down, it can exit out to the surface of the tile and be dispersed into the air. Any condensation that does not run out is so minimal it is dispersed in the air in the wall and is of no consequence.
    – RMDman
    Mar 29 at 12:38
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I have no idea what you are worried about. I never in my life have seen trim installed under tile. By doing so you would be creating an extrusion under the tile that could in fact hold water. I also don't think thinset can bind permanently to trim.

Here is the deal. Showers and kits were designed for all of this. You have a hole for your valve and stem. Every shower has it. Then you thinset and tile. Your tile can greatly overlap the backerboard (depends on the size of tile). Most of the time I give the valve about ~1/4" all the way around.

The trim goes next. I don't even caulk most of the trim I put on. The good shower kits come with good compression under and the caulk is just another place for mold and scum to hang out. I test every shower trim out. Pour water direct down the wall over the trim. Unscrew it and if it is dry nothing, if it is wet I caulk a little past the halfway point on the top half.

I have literally taken apart 100s of showers. I have never seen one that had a leak from behind the valve hole. I am positive water of some substance gets in but the cavity behind this should be rather hollow and dries (few drops here and there) almost instantly. Basically every shower/shower kits in the world are designed this way.

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  • I'm not sure why we go through all the trouble of waterproofing the cement board if a 4" hole is left unsealed. I can't say this is the answer without this fact being addressed.
    – Scott Lin
    Mar 29 at 4:20
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    Because the parts of the backerboard you are waterproofing have water directly sprayed on them. You are worried about a hole that is covered that never has water sprayed on it and has trim designed to protect it. The rest of the shower is much much more susceptible. Most shower walls leak in the last 1-2' by the floor. Waterproofing past 4' feet is useless... except the waterproofing process in general create a very tight environment where water does not dry out as good (never put insulation in your shower walls)... cont...
    – DMoore
    Mar 29 at 4:25
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    I have also taken apart many many many showers from the 40-60s that had no waterproofing and many install on drywall... with no signs of leaks or mold. They just did a great tile job and used the right type of tile/mortar. You can still do that. It isn't code because local inspectors don't trust that you will tile perfectly or keep up with tile maintenance - nor should they. You are worried about the last thing you should worry about. A hole 3-4' high with no water hitting it with a designed seal. Quit thinking about this and finish the shower to the right standards.
    – DMoore
    Mar 29 at 4:29

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